JFR: Improve check for JDK classes

In various places in the JFR code we check if an event class belongs to the JDK by checking Class::getClassLoader() == null. This works since all JDK events today are in the java.base module, but if we want to support events in other modules, for example, java.net.http, the check needs to be updated.

This would allow other modules to use mirror events, static commits and avoid unnecessary security checks.